1. Field of the Invention
The present invention relates to a process for the synthesis of perfluorobutadiene and of higher perfluoroalkadienes with terminal double bonds, by a reaction of .alpha.,.omega.-diiodo-perfluoroalkanes deiodofluorination carried out by using organometallic compounds.
2. Description of the Prior Art
Perfluorobutadiene is a chemically interesting compound, because it has been proposed for use as termonomer, in smaller amount, together with CF.sub.2 .dbd.CH.sub.2 and CF(CF.sub.3).dbd.CF.sub.2 to produce fluoroelastomers vulcanizable by peroxides (Daikin's Japanese Pat. No. 47,752 publ. July 7th, 1977). It has also been proposed for use as termonomer together with C.sub.2 F.sub.4 and trifluoronitrosomethane to manufacture another curable fluorinated elastomer (see German Pat. No. 2,304,650 and J. Chem. Soc. Perkin I, 1973, page 1111). In fact, in these copolymers, perfluorobutadiene enters by polymerizing in 1-2 position, thus leaving one double bond unaltered and available for crosslinking reactions.
Another use of perfluorobutadiene (see U.S. Pat. No. 3,980,509) is as bonding agent, in polymer form, in "fluoro-oxidizer systems", wherein such polymer acts both as bonding agent and as fuel, while a fluoride salt, such as NH.sub.4 BF.sub.4 acts as the oxidizer (see J. Appl. Polymer Sci. 19 (1975) 1359).
Another patent (U.S. Pat. No. 3,353,904) mentions perfluoropolyenes having terminal double bonds as agents to confer water-repellency characteristics to cotton.
Although perfluorobutadiene is considered as a very valuable product, the development of the products which can be obtained from it has been limited by the fact that a synthesis, suitable to be realized on full industrial scale, of such compound, was not available.
The processes known so far for the synthesis of perfluorobutadiene and of higher perfluorodienes are briefly mentioned hereunder.
In U.S. Pat. No. 3,046,304, the starting products are ICl and CClF.dbd.CF.sub.2. From the mutual reaction of these compounds CClFI--CClF.sub.2 is obtained, which in its turn dimerizes, with a yield of 82%, in the presence of an equal volume of elemental mercury, CClF.sub.2 --CFCl--CFCl--CClF.sub.2 being obtained, which in its turn can be dechlorinated by Zn powder in ethyl alcohol to perfluorobutadiene, with a yield of 98%.
This synthesis has the drawback that it can be accomplished only difficultly on industrial scale, because in the dimerization a large amount of mercury must be used, and the reaction mixture must be strongly stirred: thus, severe problems must be faced as for environmental pollution and equipment. Moreover, in the subsequent dechlorination step, the problem occurs of dispersing the Zn powder into the liquid reaction phase, which creates considerable difficulties. Finally, the first process step, i.e., the reaction of CClF.dbd.CF.sub.2 with ICl, requires extremely long reaction times (6weeks) and yields are rather low (72,6%), see C.A. 74 (1971) 126097 h.
Higher perfluorodienes, such as, e.g., 1,5-perfluorohexadiene, are obtained by starting from above mentioned CF.sub.2 Cl--CFClI, by telomerization of C.sub.2 F.sub.4 in the presence of .gamma.-radiation and under high pressure, the telomer CF.sub.2 ClCFCl(C.sub.2 F.sub.4).sub.2 I being obtained, which is subsequently chlorinated with chlorine in the presence of U.V. rays, and then dehalogenated in two steps. In the first step, accomplished with Zn powder in acetic acid+acetic anhydride mixture, a double bond is formed by dechlorination, and in the second step, carried out with Zn powder in diethylene glycol, the dechlorofluorination occurs, with the formation of the second double bond (see C.A. 74 (1971) 126097 h).
This method is complex due to the many reaction steps it requires, and its overall yield is very poor: moreover, the use of .gamma.-rays in an industrial process is complex and unproposable.
Finally, a method is known (see C.A. 98 (1983) 126788 e) for preparing perfluorobutadiene by starting from BrClFC--CBrF.sub.2, which is added (by telomerization) to ClFC.dbd.CF.sub.2 in the presence of U.V. radiation, BrF.sub.2 C--CClF--CClF--CBrF.sub.2 being obtained. This latter is dehalogenated with Zn powder in acetic acid+acetic anhydride. The method cannot be applied to higher perfluoroalkadienes. For this process too, the difficulties are considerable to an industrial implementing, because of use of U.V. rays and Zn powder. Furthermore, the synthesis of the brominated C.sub.4 intermediate occurs with low yields as referred to CClF.dbd.CF.sub.2.